Supplying or removing power, either partially or completely, from a block of circuitry may be controlled by header or footer circuits. The header circuit forms a controllable switch between a positive supply voltage and a block of sub-circuits. Similarly, the footer circuit forms a controllable switch between a negative supply voltage and the sub-circuit block. Activation of the header or footer circuits allows a virtual operating supply voltage to be connected to the sub-circuit block. Deactivation of the header or footer circuits provides a standby voltage for the sub-circuit block.
A conventional approach to providing an operating virtual voltage to the sub-circuit uses a conducting header or footer MOS transistor. Then, the forward voltage drop of a separate, external junction diode that is parallel-connected with the MOS transistor is used to provide a standby voltage for the sub-circuit when the operating voltage MOS transistor is not conducting. Alternatively, another parallel MOS transistor connected as a diode can be used to provide a standby voltage for the sub-circuit when the operating voltage MOS transistor is not conducting. However, the voltage drop obtained with either the external junction diode or the diode-connected transistor is usually not optimal, especially over a range of fabrication process variations and for different applications.
Accordingly, what is needed in the art is a more effective way of obtaining a voltage, such as a standby voltage, that is tunable and also maintains the power and area advantages of a diode-connected transistor.